Roller Finger Followers (RFF) are widely used in overhead cam internal combustion engines to sequentially open and close the cylinder intake and exhaust valves. In a typical application, the RFF serves to transfer and translate rotary motion of a cam shaft lobe into a pivotal motion of the RFF to thereby open and close an associated valve.
It is known that, for a portion of the duty cycle of a typical multiple-cylinder engine, the performance load can be met by a functionally smaller engine having fewer firing cylinders, and that at low-demand times fuel efficiency can be improved if one or more cylinders of a larger engine can be withdrawn from firing service. It is also known that at times of low torque demand, valves may be opened to only a low lift position to conserve fuel, and that at times of high torque demand, the valves may be opened wider to a high lift position to admit more fuel. It is known in the art to accomplish this by de-activating a portion of the valve train associated with pre-selected cylinders in any of various ways. One way is by providing a special two-step RFF assembly having a variably activatable and deactivatable central slider arm (also referred to herein as a high-lift follower) pivotably mounted on a follower body wherein the slider arm may be positioned for contact with a high-lift lobe of the camshaft. Such a two-step RFF typically is also configured with rollers disposed on the follower body at each side of the slider arm for contact with low-lift lobes of the camshaft on either side of the high-lift lobe. The follower body is pivotably mounted on a hydraulic lash adjuster (HLA) at one end and engages a valve stem or tappet at the other end. Thus, the two-step RFF causes low lift of the associated valve when the slider arm of the RFF is unlatched from the body in a deactivated (lost motion) position, and high lift of the associated valve when the slider arm of the RFF is latched to the body in an activated position to respond to the high lift lobe of the cam shaft.
Due to the mass moment of inertia of the high lift follower, a spring having a substantial spring load is required to ensure that continuous contact is maintained between the high-lift follower and the camshaft lobe at all camshaft rotational speeds. When the follower assembly is in valve deactivation mode and the high-lift follower is on the base circle of the cam lobe, the reaction force of the follower spring must be lower than the spring force within the HLA to permit the HLA to fully extend and eliminate mechanical lash in the valve train. If the follower spring is stronger than the HLA spring, the HLA will be gradually forced to leak down. When the amount of leak down exceeds the available volume of oil within the HLA's oil reserve chamber, the HLA will not recover and will engulf air. This causes problems with its hydraulic stiffness and can result in engine noise and error in an ensuing valve lift event.
What is needed in the art is a means that permits use of a follower spring having sufficient strength to maintain contact of the high-lift follower at all engine speeds, yet prevents compression of an associated HLA to a point of unacceptable leakdown.
It is a principal object of the present invention to maintain contact of a high-lift follower at all engine speeds without allowing compression of an associated HLA to a point of unacceptable leakdown.